A proliferation of portable battery operated electronic devices, e.g., cell phones, PDAs, electronic book readers, portable games players, MP3 music players, digital cameras, etc., have created a need for external power sources to recharge the batteries thereof. Heretofore, specific use battery chargers that plug into a standard AC power wall-mount outlet have been supplied with their respective battery operated electronic device. These battery chargers have been either a step-down transformer with or without AC to direct current (DC) rectification, or an electronic step-down voltage switch mode power supply (SMPS). The battery chargers have generally been configured in a “cube” shape with standard 15 ampere blade prongs that plug into the standard 15 or 20 ampere 120 volt AC general use wall receptacle. A DC low voltage for charging the battery is supplied to the associated electronic device by a cord and matching charger connector.
As technology has advanced and costs have been reduced for these portable battery operated electronic devices, more and more devices have been configured to connect with a Universal Serial Bus (USB) interface that is capable of supplying both data and power to the portable device. Use of a USB interface eliminates the requirement for separate power and data connectors in the portable device. In addition, the USB plug and receptacle are physically robust, and effectively isolate DC voltages from being shorted by accidental external metal contact. USB plug configured battery chargers are produced in both 120 VAC and 13.8 VDC power source voltages. By using a USB plug compatible voltage charging source, the battery in the portable electronic device may also be charged from a personal computer during data transfer operations or just when the portable device is attached to the personal computer with a standard USB cable.
Problems exist however when a USB battery charger has been misplaced, more than one portable device requires charging, and/or a personal computer is not available as a USB charger. Also some inexpensive USB battery chargers are not energy efficient, e.g., poor power factor and/or have excessive idling current draw during non-use (vampire load).